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Question

A barometer kept in an elevator reads 76 cm when it is at rest. If the elevator goes up with increasing speed, the reading will be


  1. zero

  2. 76 cm

  3. < 76 cm

  4. > 76 cm


Solution

The correct option is C

< 76 cm


Consider the barometer kept in an elevator going upwards with an acceleration a0 as shown, Consider two points A and B as the topmost and bottom most point of barometer column containing mercury. Consider the area of tube to be Δs Free body diagram of cylinder containing A & B

ℓ = length of mercury column from Newton's second law,

(PBPA)Δsmg=ma0 Where m=Δspla0 Δs(PBPA)=Δs pl(a0+g) PBPA=pl(a0+g) But PA=0 as there is vacuum above point A PB=pl(a0+g)

But if the lift was not accelerating, then pressure would have been, PB=p g l Hence the length of mercury column

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